1. Field of the Invention
The present invention relates to an engine control system and, more particularly, to an engine control system adapted to change a timing for closing an intake valve in accordance with a state of running the engine.
2. Description of the Related Art
Japanese Patent Unexamined Publication (kokai) No. 63-195,325 discloses an internal combustion engine adapted to lower an effective compression ratio by retarding the timing for closing an intake valve of the engine.
Japanese Patent Examined Publication (kokoku) No. 2-36,772. discloses a so-called lean burn engine adapted to set an absolutely lean air-fuel ratio in which the air-fuel ratio is made leaner than the stoichiometric air-fuel ratio.
It should be noted that the lean burn engine is required to make a compression ratio higher in order to ensure a high degree of ignitability. However, when the compression ratio of the engine is made higher, the problem may arise that the higher the compression ratio, the more difficult it becomes to start the engine. More specifically, when an attempt is made to restart the engine in a warm state after it has once been suspended temporarily, it is necessary to further compress the highly pressurized gases existing in the cylinders. Accordingly, to restart the engine, a large amount of force is required so that a higher power is needed in order to set the higher compression ratio. This problem may occur more or less even in a relatively low compression ratio.